1. Field of the Invention
The invention relates to a method and an apparatus for linearity measurement in a reproduction appliance that contains an operating carriage bearing a scanning or recording device and that can be moved in discrete steps along a straight line.
Reproduction appliances that contain an operating carriage having a scanning or recording device are, for example, scanners, engraving machines, or exposers for printing plates, films, foils, paper, and so on. The drive to the operating carriage is generally provided through force transmission elements that are connected by a form fit, for example, spindles, racks, toothed belts, and/or gear wheels. Such force transmission elements have production-induced errors, which can impair the linearity of the advance travel, so that they have to be taken into account in the high resolution demanded of reproduction appliances.
For instance, in the case of exposers, a linearity of less than 0.01% (1xc3x9710xe2x88x924) is required for both directions of the exposure. Thus, for a distance of one (1) meter, the linearity deviation must not be more than +/xe2x88x9250 xcexcm. To achieve such accuracy in the advance direction of the operating carriage, one needs highly precise force transmission elements such as transport spindles, which operate on the circulating ball principle and have a pitch error of less than 1 xcexcm per revolution. Such drive elements, such as those for the transmission of forces in precision machine tools, are very costly. In addition, they are over dimensioned for the use in reproduction appliances because the forces to be transmitted in reproduction appliances are relatively small. For the low-torque drive motors in exposers, they even have, with their configuration-induced torque fluctuations, considerable disadvantages.
In the case of reproduction appliances, it would intrinsically be possible to use simpler and, therefore, less accurate drive elements if the linearity error were measured and the drive to the reproduction appliances were corrected appropriately during operation. Methods of length measurement that supply the necessary accuracy are, for example, registering measured values by interferometers or glass scales. However, their resolution would have to be chosen to be significantly higher than the demanded measurement accuracy because these methods have superimposed systematic errors that exceed the theoretically achievable resolution many times over. In addition, such high-resolution length measurement systems are extremely complicated in terms of manufacture and application.
It is accordingly an object of the invention to provide a method and apparatus for linearity measurement that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type such that a simple and cost-effective linearity measurement is made possible in a reproduction appliance.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for linearity measurement in a reproduction appliance, including the steps of providing an operating carriage bearing at least one of a scanning device and a recording device, successively moving the operating carriage along partial segments of a straight line in discrete steps in a given direction along the straight line, each of the partial segments being longer than a travel corresponding to one of the steps and being shorter than the entire length of the line, the partial segments each having an exactly defined length defined by an additional carriage moveable along the given direction, counting a number of steps needed for each partial segment, and calculating deviations between values for a travel of the operating carriage resulting from the partial segment and values for the travel of the operating carriage resulting from counting the steps.
In that the necessary devices are integrated into the reproduction appliance, or are already present therein, the method can be carried out completely within the machine. Alternatively, a suitable measuring tool can be inserted into the machine and, after the recording and storage of the data, is removed again. The evaluation of the data can be carried out either in the reproduction appliance or outside it, for example, on a personal computer.
The counting of the discrete steps is available in any case in a reproduction appliance that has an incremental drive for the operating carriage, for example, a stepping motor, so that the internal machine measuring increments can additionally be used to carry out the method according to the invention. Otherwise, the counter values can be easily obtained electronically, for example, by an incremental encoder that is connected to the drive. It is, therefore, possible for internal machine measuring increments that are already present, such as the steps from a stepping motor or rotary encoder cycles, to be used additionally to carry out the method.
The only mechanical parts that have to be fabricated and adjusted precisely are the additional carriage and its drive. However, these precision requirements can be fulfilled without great expenditure because the length defined with the aid of the additional carriage is substantially shorter than the entire travel of the operating carriage.
In accordance with another mode of the invention, at the start the operating carriage is moved to the start of the line, and the additional carriage is moved into a first relative position between the operating carriage and the additional carriage. The additional carriage is then moved into a second relative position between the operating carriage and the additional carriage, the distance between the first and the second relative position corresponding to the length of a partial segment, also referred to as a part line. Next, the operating carriage is shifted forward while the discrete steps are counted and while the additional carriage remains in place. The operating carriage is stopped as soon as the first relative position between the operating carriage and the additional carriage has been reproduced. The additional carriage is then moved again into the second relative position while the operating carriage remains in place, and the alternating forward shifting of operating carriage and additional carriage is repeated until the entire line has been measured.
In accordance with a further mode of the invention, the part lines that are traveled over successively preferably all have the same length. As a result, both the definition of the precisely defined lengths and the evaluation of the measured results are made easier. Specifically, in accordance with added modes of the invention, the two relative positions between the operating carriage and the additional carriage can be defined or produced in a very simple way such as with mechanical stops, electromechanical drives, and/or optical sensors. The first relative position can be defined, for example, with the aid of a stop, while an optical device such as a differential light sensor are rather more suitable for defining the second relative position.
In accordance with an additional mode of the invention, the additional carriage is moved into the first relative position with an electromechanical device before traveling over each partial segment. The electromechanical device moves the additional carriage into the first relative position before traveling over each partial segment, the electromechanical device being connected to the additional carriage.
In accordance with yet another feature of the invention, the additional carriage is held in the first relative position with one of a self-locking device and self-retaining device while the operating carriage is shifted forward.
The additional carriage preferably has a self-locking device in order to remain in the first relative position when the operating carriage is shifted forward. Alternatively, an active brake or other blocking device can be provided for the additional carriage.
In accordance with yet a further mode of the invention, at least one of a light source and a reflector is fixed to one of the operating carriage and the additional carriage, and at least one of a differential light sensor and a reflector is fixed to another one of the operating carriage and the additional carriage.
In accordance with yet an added feature of the invention, the operating carriage is moved along the straight line with an elongate threaded spindle, preferably, a roller-burnished spindle, having a spindle pitch, the deviations between values for the travel of the operating carriage resulting from the partial segments and values for the travel of the operating carriage resulting from counting the steps corresponding to deviations between actual values of the spindle pitch and desired values of the spindle pitch.
The additional carriage is preferably guided on the linear guide that is present in any case for the operating carriage, but it can also have a dedicated guide track. Under certain circumstances, a dedicated guide track for the additional carriage may be more beneficial, for example, if the additional carriage represents a measuring tool that is inserted into the reproduction appliance to carry out the method and is subsequently removed again. In such a case, the measuring tool can also be used for linearity measurement on other machines.
The method according to the invention is so uncomplicated and can be carried out so simply that reproduction appliances can be equipped as standard with the additional carriage. As a result, when the reproduction appliance is switched on and/or on request by an operator of the reproduction appliance, self-balancing can be carried out, by the calculated deviations between the values for the travel of the operating carriage resulting from the partial segments and the values for the travel of the operating carriage resulting from counting the steps being used to correct the drive of the operating carriage during subsequent operation of the reproduction appliance.
In accordance with yet an additional feature of the invention, the spindle is rotated in discrete steps corresponding to constant rotational angles of the spindle.
In accordance with again another feature of the invention, the spindle is rotatably driven with a stepping motor.
With the objects of the invention in view, in a reproduction appliance containing an operating carriage bearing at least one of a scanning device and a recording device, the operating carriage being able to be moved in discrete steps in a given direction along a straight line, there is also provided an apparatus for linearity measurement including an additional carriage linearly moveable in the given direction between a first relative position and a second relative position having a position spacing corresponding to partial segments with an exactly defined length, each of the partial segments longer than a travel corresponding to one of the steps and shorter than the entire length of the line, and a device for calculating deviations between values for the travel of the operating carriage resulting from the partial segments and values for the travel of the operating carriage resulting from counting the steps.
In accordance with again another feature of the invention, there is also provided a drive connected to the operating carriage for driving the operating carriage, the drive having an elongate threaded spindle with a spindle pitch, the deviations between values for the travel of the operating carriage resulting from the partial segments and values for the travel of the operating carriage resulting from counting the steps representing deviations between actual values of the spindle pitch and desired values of the spindle pitch.
In many reproduction appliances, the operating carriage is moved by an elongate threaded spindle, which is driven in rotation by a drive motor. The deviations between the values for the travel of the operating carriage resulting from the partial segments and the values for the travel of the operating carriage resulting from counting the steps in such a case correspond to deviations between the current values of the spindle pitch and the desired values of the spindle pitch.
In the case in which the operating carriage is moved through a rack, a toothed belt, or the like, the method supplies deviations between the current values of the tooth pitch and the desired values of the tooth pitch.
The method according to the invention is suitable, for example, for reproduction appliances whose operating carriage is driven by force transmission elements connected by a form fit, such as spindles, racks, toothed belts and/or gear wheels that, although they have no slip, have reproducible and repeated, production-induced linearity errors including pitch or graduation errors, which can be both periodic and aperiodic.
Because these errors can be balanced out during subsequent operation of the reproduction appliance by appropriate correction of the drive, the force transmission elements when carrying out the method according to the invention merely have to satisfy the requirement that they produce repeated errors. For example, it is possible to use a simple roller-burnished spindle.
The method according to the invention is also suitable for reproduction appliances whose operating carriage is driven in any other way than by mechanical force transmission elements, provided these have repeated errors that can be corrected. Such a drive would, for example, be an electromagnetic linear motor whose rotor is fixed to the operating carriage.
If a spindle is used to advance the operating carriage, it is beneficial if the spindle is driven in rotation by a stepping motor. Stepping motors are cost-effective and can be obtained with very high-resolution incrementing of the revolution and also negligible systematic errors, comparable in qualitative terms with rotary encoders and timing disks.
For example, in the case of a spindle, in accordance with a concomitant feature of the invention, it is advantageous if the length of the partial segments that are traveled over successively during a measuring operation can be selected from at least two different predefined values, of which at least one is substantially smaller than the spindle pitch and at least one is substantially greater than the spindle pitch. As such, short-period pitch errors, that is to say errors within one spindle revolution, and/or long-period pitch errors, which extend over a plurality of spindle revolutions, can be registered as exactly as possible if desired. For gear drives, this applies in a corresponding way in relation to the tooth pitch.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and apparatus for linearity measurement, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.